An important influence on climate is mountainousterrain, especially the long, high chains of mountainsthat act as climatic divides. These obstacles deflect thetracks of cyclones and block the passage of air massesat the lower levels. If the pressure gradients are strongenough to force the air masses over the mountains, theforced ascent and descent modifies the air masses to agreat extent, thus modifying the climate on both thewindward and leeward sides of the range.The orientation of the mountain range may blockcertain air masses and prevent them from reaching thelee side of the mountains. For example, the Himalayasand the Alps, which have east-west orientations,prevent polar air masses from advancing southward.Therefore, the climates of India and Italy are warmer inwinter than are other locations of the same latitude. Thecoastal ranges in North America, running in anorth-south line, prevent the passage of unmodifiedmaritime air masses to the lee side.The most noted effect of mountains is thedistribution of precipitation. The precipitation values,level for level, are much higher on the windward sidethan on the leeward side.In regions where the prevailing circulation flowsagainst a mountain barrier, the amount of precipitationincreases more or less uniformly with elevation on thewindward side of the range. This steady increasenormally occurs up to elevations of about 10,000 feet.However, in the trade wind zone (such as at theHawaiian Islands), precipitation increases only to about3,000 feet and then decreases gradually. Even with thisdecrease in amount, more rain is received at 6,000 feetthan at sea level.Another important topographical feature is thepresence of lakes. The lake effect can be notable forlarge unfrozen bodies of water. The lee sides of lakesshow considerable diurnal and annual modification inthe form of more moderate temperatures; increasedmoisture, clouds, and precipitation; and increasedwinds (due to less friction) and land and sea breezeeffects.OCEAN CURRENTSOcean currents play a significant role in controllingthe climate of certain regions. Ocean currents transportheat moving cold polar water equatorward into warmerwaters and moving warm equatorial water polewardinto cooler waters.Currents are driven by the major wind systems;therefore, cold southward-moving currents flow alongthe west coasts of continents, and warm northwardmoving currents flow along the east coasts ofcontinents. This is true in both hemispheres. Basically,this results in cooler climates along the west coasts andwarmer climates along the east coasts.A brief explanation of the effects of ocean currentsis presented here.Effects on the West CoastsThe northern portions of the west coasts ofcontinents generally have cool summers and warmwinters. The summers are cool because of the presenceof cold northern waters along their shores. However,the winters are generally mild because of the transportof warm ocean waters to these latitudes. For example,the south and southeast coasts of Alaska and the westcoasts of Canada, Washington, and Oregon haverelatively warm currents flowing along their shores.These currents are the Aleutian and North Pacificcurrents,whicharebranchesofthewarmnorthward-flowing Kuroshio Current. The currentsflow along the West Side of the Pacific high and bringwarm water into southern Alaska and the PacificNorthwest.As these currents merge and flow southward alongthe British Columbia coast, they move into warmerwaters and become the cold California Current.The southern portions of the west coasts ofcontinents generally have cooler climates than do theeast coasts of the same latitude. For example, duringsummer, the cold California Current flows southwardalong the shores of California. Due to the Pacific high,the winds normally flow either across the cold currenttoward shore (onshore) or parallel to the coastline. Thisresults in cool air being advected inland allowing citiessuch as San Francisco and Seattle to enjoy relativelycool summers. Unfortunately, when the warm, moist airfrom the Pacific high does move over the underlyingcold current, extensive fog and stratus develop whichalso move inland. This situation is typical along thesouthernportionsofthewestcoastsinbothhemispheres.Another factor affecting west coasts is upwelling.Upwelling is the process by which cold subsurfacewaters are brought to the surface by wind. It occurs inareas where the wind causes the surface water to betransported away from the coast. The colder subsurfacewater then replaces the surface water. In the Northern6-11